The high acid values common in rosins utilized to make ink resins can be lowered by reacting the rosins with zinc oxide, calcium acetate, or similar compounds to produce metallic resinates. Metal resinates are widely used as binders in gravure printing inks due to their rapid solvent release and their ability to prevent some basic pigments from livering. These resinates are soluble in hydrocarbons, but not in alcohol.
One of the shortcomings of using metal resinates in gravure inks has been their low solution viscosities, a byproduct of their low molecular weights. These low viscosities make it difficult to formulate inks having the desired pigment-to-binder ratios at press viscosities that are necessary for this type of printing.
Another critical property of metal resinate solutions that is directly linked to viscosity is dilutability. Viscosity is measured by the time required for an exact quantity of solution to flow by gravity through a specially sized apparatus. Dilutability is measured by the amount of solvent needed to reduce the viscosity of a given weight of resinate solution to a certain level. The typical specification for a metal resinate solution calls for the volume of toluene needed to reduce 100 grams of resinate to an 18 second flow as measured with a #2 Shell cup.
The usual dilutability values of commercial resinates are between 70 to 120 ml. Ink makers would like dilutability values higher than this in order to achieve a desirable balance of solids, color strength, and viscosity in finished inks.
To solve these problems of viscosity and dilutability with metal resinate formulations, ink makers traditionally have added small amounts of ethylcellulose or ethylhydroxyethyl-cellulose (EHEC) to the ink as a thickener (the properties of which are discussed in U.S. Pat. No. 2,610,180). EHEC owes its thickening ability to three factors: (a) a very high molecular weight, (b) a rigid molecular structure, and (c) intermolecular association via hydrogen bonding of unetherified hydroxyl groups on the cellulose backbone. These factors permit EHEC to be used to produce a drastic reduction in resin solids at press viscosity (expressed in the industry as a "high dilution").
However, using EHEC as a thickener gives rise to other problems, as described in Leach, R.H., The Printing Ink Manual, Van Nostrand Reinhold Co. Ltd., London, 1988. Very small amounts of EHEC can cause substantial reductions in the gloss of an ink. Also, the cost of EHEC is relatively expensive. Finally, EHEC is only marginally compatible with metal resinates. This incompatability seems to vary from batch to batch, and may manifest itself in phase separation of the ink upon standing.
Attempts have been made to eliminate those problems with EHEC by producing other ink thickeners. It is known to prepare printing ink binders based on reaction products of (a) natural resins, (b) other synthetic resins (for example hydrocarbon resins having a bromine number of 5 to 80), and (c) calcium compounds (and, if appropriate, other compounds of group II of the periodic system). In this process an unsaturated dicarboxylic acid (i.e., maleic anhydride) is reacted at the same time. This method, taught in U.S. Pat. No. 3,468,829, has a disadvantage in that the resulting viscous products are virtually not utilizable as printing ink binders.
Other known binders are taught in U.S. Pat. No. 4,528,036 and U.S. Pat. No. 4,552,592. Here, the binders are based on the reaction products of (a) natural resins, (b) a copolymer or a synthetic resin and (c) calcium compounds. The resulting reaction products are reacted subsequently with acetic acid to form a salt of the corresponding resin. These solutions are of relatively low molecular weights and viscosities.
Yet another known binder is the product complex formed by reaction of a metal resinate and an amine-reactive polyamide as taught in U.S. Pat. No. 4,767,835. The amine-reactive polyamide, being a condensation polymer, is of comparatively low molecular weight.
An example of a dilution enhancer which utilizes a nitrogen-zinc complex mechanism similar to the one operative in the present application is taught in the commonly assigned U. S. Pat. No. 07/571,990 filed by G. Frederick Hutter on Aug. 24, 1990. This previously filed application teaches that resinate dilutability can be substantially enhanced by the use of small amounts of acrylic polymers containing pendant amine groups. These amine groups form coordinate complexes with the zinc ions present in the resinate. These formed complexes create a higher apparent molecular weight which provides higher viscosities to the resinate, thereby allowing desired dilution of the inks with solvent prior to printing. When evaluated against EHEC, the disclosed aminoacrylate resins were comparable in thickening ability and did not produce the gloss reduction problems associated with the use of EHEC. However, these resins have proven to be more difficult to blend with resinates than EHEC due to gel formation that can be reduced only by vigorous agitation.
Despite the development of these, and other, thickeners EHEC remains the preferred viscosity enhancer in the field. Therefore, it is an object of this invention to produce an economical thickener of a high molecular weight and high dilutability, which exhibits an enhanced compatibility with the metal resinates commonly utilized in gravure printing inks.